1. Field of the Invention
The present invention generally relates to a memory unit, in particular, to a non-volatile memory unit.
2. Description of Related Art
Random access memory devices are mainly divided into dynamic random access memory (DRAM) and static random access memory (SRAM). The advantages of SRAM include faster operation and lower power consumption. Furthermore, compared with DRAM, the SRAM requires no periodic refreshing of the data and has a simpler design and fabrication process. As a result, SRAM has been widely adopted in many kinds of information electronic products.
Nevertheless, SRAM is a kind of volatile memory that relies on the conductive state of a transistor inside the memory cell to save data. Therefore, when the power supply to the memory is shut down, the saved data inside the SRAM will be lost.
On the other hand, electrically erasable programmable read only memory (EEPROM) allows multiple data writing, reading and erasing operations. Furthermore, the saved data will be retained even after power to the device is removed. With these advantages, EEPROM has been broadly applied non-volatile memory devices in personal computer and electronic equipment.
However, with continuous advance in technologies and the techniques of fabricating integrated circuits, information electronic products such as computers, mobile phones, digital cameras or personal digital assistants (PDA) are increasingly common. Although the amount of data that need to be processed and stored inside the information electronic product continue to increase, the basic demands for a light, portable, compact body must also be considered at the same time. Therefore, to prevent the lost of memory data when power supply to the SRAM is shut down, finding an innovative design of a semiconductor device having the advantages of high operating speed of SRAM and high storage capacity of flash memory is high on the developing list.